The present invention is directed to an improved catalyst support and to the resultant catalyst suitable for treating exhaust products from internal combustion engines, especially diesel engines. The support of the present invention provides a means of inhibiting the degradative effects on exhaust gas conversion catalysts associated with sulfur and sulfurous compounds found in combustion engine exhaust products.
The exhaust products of internal combustion engines are known health hazards to human beings, animals as well as plant life. The pollutants are, in general, unburnt hydrocarbons, carbon monoxide, nitrogen oxides, as well as residual amounts of sulfur and sulfurous compounds.
Exhaust catalysts have to meet stringent requirements with respect to light-off performance, effectiveness, long-term activity, mechanical stability as well as cost effectiveness in order to be suitable for vehicle application. For example, exhaust catalysts must become active at low operating temperatures initially encountered under start-up conditions and, at the same time, must be able to cause high percentage of conversion of the pollutants under varying temperature and space velocity ranges encountered during normal operation.
The pollutants of unburnt hydrocarbons, carbon monoxides as well as nitrogen oxides have been successfully treated by contact with multifunctional, noble metal catalysts which are capable of converting a high percentage of the pollutants into less harmful products of carbon dioxide, water (steam) and nitrogen. However, the sulfur and sulfurous compounds present in fuels and, in turn, in exhaust product have been known to poison the noble metals resulting in lessening their catalytic effectiveness and life. Recently, fuels useful for internal combustion engines have come under stringent regulations with respect to their content of sulfur and sulfur containing compounds. However, complete removal of these materials, especially from middle cut petroleum feedstock (C10 and greater hydrocarbons), is difficult to accomplish due to the complex nature and scope of such compounds. Thus, sulfurous materials are present in fuels for internal combustion engines, especially in fuels for diesel fuel combustion engines.
The “catalytic converter” used to convert the harmful pollutants into non-harmful gases, usually consists of three components—the catalytically active metal, the support on to which the active metal is dispersed, and a substrate on to which the support is applied or “washcoated”.
The catalytic metals that are useful to cause effective conversion of harmful pollutants, like carbon monoxide, nitrogen oxides, and unburnt hydrocarbons under the varying conditions encountered, are noble metals, usually the metals of the platinum group, such as platinum, palladium, rhodium and mixtures thereof. These noble metal catalysts are described in DE-05 38 30 318, which is incorporated herein in its entirety by reference.
The noble metals are conventionally supported on high surface area oxides, such as aluminum oxide (or alumina). The high surface area alumina is applied or “washcoated” onto a ceramic or metallic substrate, such as in the form of a honeycomb monolith or wire mesh or the like structure. It is also possible to apply the noble metals onto the support after washcoating the support material onto the monolith.
Various forms of alumina are commonly used as high surface area support material for emission control catalysts because of their availability, ease of formation, thermal properties and their ability to promote the catalytic activity of the noble metal. However, the disadvantage of alumina supports are that they adsorb sulfur and/or sulfurous compounds, as found in the engine emission stream. When so adsorbed, the sulfurous compounds are known to poison noble metal catalysts, especially those formed with platinum metal, causing reduction in activity and effective life of the catalyst system.
In contrast to alumina, silica supports are known not to interact with sulfur and sulfurous compounds. Therefore, noble metal catalysts formed with silica as the high, surface area support material do not exhibit poisoning by sulfur and sulfurous compounds. However, silica does not exhibit the hydrothermal stability required to form effective emission control catalyst supports and, therefore, is not a desirable catalyst support material for such applications.
In attempts to overcome the poor hydrothermal properties associated with silica, it has been added to alumina supports either by standard impregnation or co-precipitation techniques (See U.S. Pat. No. 2,804,433). In both instances, the resultant support retains a significant amount of exposed alumina and, therefore, these supports still exhibit a high degree of susceptibility to sulfur poisoning. Further, the applied silica tends to precipitate in the pores of the alumina causing a reduction in porosity to, thus, reduce the surface area upon which the noble metal can reside.
U.S. Pat. No. 3,925,253 discloses an alumina support that has been impregnated with silica by applying an aqueous solution or sol of silica to the alumina support material. Although this material exhibits good attrition resistance, the alumina remains exposed and susceptible to poisoning by the sulfur in the emission product. It is known that conventional impregnation techniques permit the silica to precipitate or agglomerate (due to its tendency to favor homogenous nucleation of discrete silica particles) to form a discontinuous, patch-like coating of irregular thickness on the surface of the alumina.
U.S. Pat. No. 3,923,692 discloses an alumina support, in the form of beads, spheres, or extrudates, which is first contacted with an agent capable of solubilizing the alumina at the particles' surface. The thus treated alumina structure is then impregnated with a silica solution to form a mixed silica-alumina surface. The resultant support is taught to have enhanced activity as a hydrodesulphurization catalyst.
Similarly, RE 29,771 discloses a sulfur adsorbent composed of alumina support having silica coating to enhance the strength of the material. The reference states that silica should not be used in greater than 5% in order to leave alumina exposed to cause sulfur dioxide to be adsorbed. Thus, the teaching directs one to retain a large amount of exposed alumina surface.
Alternately, support materials have been formed by co-precipitation of silica and alumina precursors to achieve a mixed support product.
U.S. Pat. No. 6,399,530 discloses a silica-alumina co-precipitated product, which further contains a Y-type zeolite for hydrocracking of petroleum. The silica-alumina product has enhanced attrition properties in comparison to alumina supported zeolite catalysts. Such a product would have large amounts of exposed alumina which may interact with sulfur containing materials.
U.S. Pat. No. 3,269,939 discloses a noble metal catalyst using a co-precipitated silica-alumina support in which silica comprises from 75 to 90 wt. percent of the support. Because of the large content of silica throughout the formed support, it is taught to have high tolerance to the presence of sulfur. However, such material would exhibit poor hydrothermal properties.
U.S. Pat. No. 3,703,461 discloses a silica-alumina support formed from an alumina gel into which a silica-alumina cogel is dispersed. The noble metal is then added using an ion exchange process.
Although U.S. Pat. No. '939 and U.S. Pat. No. '461 both provide a support having enhanced sulfur tolerance, the materials exhibit poor hydrothermal properties.
U.S. Pat. No. 6,288,007 discloses an alumina support having its surface treated with silica either by impregnation or from a gas phase deposition. The impregnation is conducted by first forming a solution of a silicon compound, contacting the alumina support with the solution and then, at elevated temperature, treating the support with air or water to convert it to an oxide.
It is desired to form an alumina catalyst support capable of enhancing the activity of noble metals in the conversion of carbon monoxide and hydrocarbon materials to carbon dioxide and water while exhibiting high tolerance to the presence of sulfur and sulfurous compounds.
It is further desired to form an alumina catalyst support having a minor amount of silica clad on the support's surface while exhibiting high efficiency to tolerate the presence of sulfur and sulfurous compounds. Catalysts formed with the silica clad alumina support of the present invention are capable of converting noxious emission products of internal combustion engines, especially diesel engines, to more environmentally benign products. Such catalysts exhibit higher activity over an extended life because of its enhanced efficiency to tolerate the presence of sulfurous compounds.